High-Concentration Carburized/Low-Strain Quenched Member and Process for Producing the Same

ABSTRACT

A super carburized, low-distortion quenched member with higher performance and minimized heat-treatment distortion is provided. A process for the production thereof includes a primary treatment and a secondary treatment. The primary treatment includes heating a steel member for a machine structure to a temperature within an austenite region by vacuum carburizing (low-pressure carburizing) to have carbon dissolved at least at a eutectoid carbon concentration of a surface layer portion of the member and then quenching the member to have at least one of ultrafine carbide and nuclei of the carbide formed in the surface layer portion of the member. The secondary treatment includes subsequently heating and soaking the member to a temperature within the austenite region and then conducting rapid quenching to have ultrafine carbide precipitated in an outermost surface layer portion.

DESCRIPTION

1. Technical Field

This invention relates to carburizing and quenching treatment widelyused as a reinforcement method for machine structural members, morespecifically to a super carburized, quenched member featuring tempersoftening resistance, high strength, high contact pressure and the like,especially to a super carburized, low-distortion quenched member (whichmay hereinafter be referred to simply as “member”) with mutuallyconflicting properties, that is, higher performance and heat-treatmentdistortion attained together and also to its production process.

2. Prior Art

Owing to excellent properties such as high fatigue strength and wearresistance, carburized and quenched members (hereafter referred to as“case hardened members”) are widely used as various members in transportequipment, industrial machines and the like. From the viewpoint ofdimensional reductions, weight reductions and/or the like throughfurther improvements in the performance of such members, numerousdevelopments have been made on case hardened members. Recently, thevacuum carburizing (low-pressure carburizing) process has beendeveloped. Compared with the conventional gas carburizing process, thevacuum carburizing process has excellent characteristic features such asenvironmental friendliness, the prevention of intergranular oxidation,the feasibility of high-temperature carburizing treatment, and easycontrol of carburizing and carbon diffusion, and therefore, is expectedto find still broader utility from the standpoints of furtherimprovements in the performance and quality of members and furtherimprovements in their productivity.

As a method for providing a machine structural member such as a gear oraxle member with improved pitting resistance by applying carburizing andquenching to the member, there is carbonitriding treatment. According tothis treatment, carbon and nitrogen are caused to concurrently diffuseinto the matrix of a member such that the member can be provided withimproved temper softening resistance. In addition, there has also beendeveloped super carburizing treatment to have carbide precipitated in asurface layer portion of a member such that the member can be providedwith improved temper softening resistance. Keeping in step withevolutions in low-pressure carburizing facilities, a great deal ofresearch has been conducted in recent years.

As a representative example of the super carburizing treatment, PatentDocument 1 discloses a carburizing treatment process for a member.According to Patent Document 1, it is proposed to form quasispheroidalor spheroidal carbide at a volume percentage of 30% or higher within arange up to a depth of 0.4 mm by conducting precarburizing to such acarbon content that spheroidal carbide is caused to precipitate in asurface layer portion of a steel member and the carbon concentration inthe surface layer portion becomes not higher than Acm but not lower thana eutectoid concentration between steel and carbon, slowly cooling orquenching the thus-treated member to convert the surface layer portioninto a bainite, pearlite or martensite structure, and then heating themember at a ramp rate of not greater than 20° C./min from the Ac1 pointto a temperature in a range of from 750 to 950° C. to effect carburizingand quenching.

According to the above-described process, the member can be improved inproperties such as pitting properties owing to the precipitation of thecarbide in the surface layer portion of the member. Nonetheless, theresulting member involves problems such as a deformation and distortionby heat treatment, because the process is super carburizing that causesthe precipitation of the carbide as much as 30% in the surface layerportion.

As a method for causing carbide to precipitate in an ultrafine form in asurface layer portion of a member by super carburizing, many heating andcooling methods have been investigated. In Patent Document 1, it isdescribed to be desirable that subsequent to the precurburizing, aircooling (which forms a bainite or pearlite structure) or quenching(which forms a martensite structure) is conducted, and that in thecarbide-forming treatment as the next step, the member is heated at aslow ramp rate of not greater than 20° C./min from the Acltransformation temperature to a temperature within the range of from 750to 950° C., and after direct quenching or air cooling, the member isagain heated and quenched.

Further, Patent Document 2 and Patent Document 3 propose, as an optimalmethod, to conduct slow cooling (or 30° C./hr or less) afterprecurburizing or primary carburizing.

When the quenching after the precarburizing or primary carburizing isconducted by air cooling or slow cooling in the method disclosed inPatent Document 1, 2 or 3, however, a network of carbide tends toprecipitate along grain boundaries in a surface layer portion of amember. The next step, that is, the carbide-forming treatment can hardlybreak up the network of carbide in a short time to have the carbidedistributed and precipitated within the surface layer portion. Toovercome this shortcoming, heating and subsequent cooling may beconducted a plurality of times in some instances.

On the other hand, Patent Document 1 also discloses quenching with anaim directed toward forming a martensite structure by increasing thecooling rate of a member subsequent to its precarburizing. Thistechnique, however, involves a potential problem that carbide nuclei ina surface layer portion may dissolve out. It is also concerned that thequenching may take place with supersaturated carbon, and due tohigh-carbon martensitic transformation, the member may develop a greaterdeformation or distortion through an expansion, shrinkage or the like.

Patent Document 4 discloses a production process of a case hardenedmember by low-pressure carburizing. There is a reference to theconversion of carbide into an ultrafine form such as the control of thecarbon concentration at 0.5 to 0.7 wt. % in primary carburizing and at0.7 to 1 wt. % in secondary carburizing and the control of primarycooling at a very slow rate of from 1 to 10° C./min. Concerningdeformation strain, however, this production process is not expected tobe preferred like the above-mentioned Patent Documents 1, 2 and 3.

Just for readers' information, a description is now made of someadvantages of low-pressure carburizing, which is finding wide-spreadcommercial utility in recent years, over conventional gas carburizing.

-   a) A change from a carburizing step to a diffusion step can be    readily and promptly modified.-   b) High-temperature treatment is feasible so that prompt carburizing    can be conducted.-   c) No intergranular oxidation takes place in a surface layer portion    of a member, and in the member under treatment, it is hence possible    to inhibit the occurrence of cracks which would otherwise begin to    take place from such a defect.-   d) No sooting takes place, thereby causing no uneven carburizing    which would otherwise take place as a result of sooting.

Patent Document 1: JP-B-62-24499 Patent Document 2: JP-B-2787455 PatentDocument 3: JP-B-2808621 Patent Document 4: JP-A-2002-348615 DISCLOSUREOF THE INVENTION Problem to be Solved by the Invention

Even in super carburizing by the conventional low-pressure carburizing,however, no optimal balance can be achieved between the progress offormation of carbide within a surface layer portion of a member undertreatment and the microstructure of the surface layer portion. Theproblem of a deformation or strain of the treated member, therefore,still remains unresolved. As a consequence, grinding, strain-correctingfinishing or the like is essential for the member after the carburizingstep. Such additional work has led to a reduction in the inherentability of super carburizing that permits use under higher contactpressure, a reduction in productivity and an increase in manufacturingcost, thereby preventing the popularization of super carburizingtreatment. Means for Resolving the Problem

The present invention has resolved the above-described problem bydeveloping an optimal process, which makes it possible to use a memberunder a higher contact pressure and also to provide the member with alower strain while making use of low-pressure carburizing facilitiesthat permit a variety of control promptly with higher accuracy as to theconcentration of carbon in the member, the repetition of carburizingtreatment/diffusion treatment, and diverse temperature conditions,heating conditions and cooling rate (quenching) conditions for heating,soaking, carburizing, quenching and the like of the member.

The above-described problem can be resolved by the present invention asdefined below:

-   -   1. A process for producing a super carburized, low-distortion        quenched member, which comprises a primary treatment of heating        a steel member for a machine structure to a temperature within        an austenite region by vacuum carburizing (low-pressure        carburizing) to have carbon dissolved at least at a eutectoid        carbon concentration of a surface layer portion of the member        and then quenching the member at a cooling rate of from 3 to 15°        C./sec from the temperature within the austenite region to a        temperature not higher than an A₁ transformation point to have        at least one of ultrafine carbide and nuclei of the carbide        formed in the surface layer portion of the member; and a        secondary treatment of subsequently heating and soaking the        member to a temperature within the austenite region and then        conducting rapid quenching to have ultrafine carbide        precipitated in a range of from 10 to 30% in terms of effective        hardened depth percentage in an outermost surface layer portion.    -   2. A production process as described above, wherein in the        secondary treatment, additional carburizing treatment is applied        to the surface layer portion of the member.    -   3. A production process as described above, wherein in the        secondary treatment, the ultrafine carbide is caused to        precipitate in the surface layer portion of the member to form a        structure composed primarily of martensite and containing a        mixed structure of troostite and retained austenite or the like        in parts thereof such that the outermost layer portion (a        portion A) of the layer, a layer portion (a portion B) inner        than the portion A and a layer portion (a portion C) inner than        the portion B are in an order of A≧C≧B in terms of the fineness        of austenite grain size.

A super carburized, low-distortion quenched member comprising a surfacelayer portion of a structure composed primarily of martensite andcontaining a mixed structure of troostite and retained austenite or thelike in parts thereof, wherein in the surface layer, an outermostsurface layer (a portion A), a layer (a portion B) inner than theportion A and a layer (a portion C) inner than the portion B are in anorder of A≧C≧B in terms of the fineness of austenite grain size.

Advantageous Effects of the Present Invention

The process according to the present invention performs the treatment ofa member in low-pressure carburizing facilities while making thecombined use of the primary treatment of conducting adequate supercarburizing and quenching at an optimal cooling rate and the secondarytreatment of subsequently causing a fine carbide to simply andefficiently precipitate; and can minimize the deformation and strain ofthe member treated through the heat treatment. Owing to the adoption ofthis process, the greatest concern about the conventional supercarburizing, for example, the cumbersome grinding, strain correction andthe like of the member after the treatment, such as the bending of anaxle or the deformation strain of a tooth profile, can be substantiallyrelieved, thereby bringing about advantageous effects that significantimprovements can be made in the productivity, quality and cost of thecase hardened member.

According to the process of the present invention, additionalcarburizing treatment may be applied to the surface layer portion of themember in the secondary treatment. This additional carburizing treatmentmakes it possible to achieve a high hardness of matrix and also toreduce the crystal grain size of an outermost surface layer portion ofthe member to an ultrafine grain size and, therefore, is also extremelyeffective for providing the member with higher strength and highertoughness. By the process of the present invention, it is possible toreadily achieve higher strength, higher toughness, higher contactpressure and the like for members such as axles and gears to which supercarburizing has heretofore been hardly applicable. Therefore, theprocess according to the present invention can be widely applied tofields where there is a high need for such properties, and has anadvantageous effect that it can make significant contributions toimprovements in the performance of a member and also to reductions inthe size and weight of the member.

BEST MODES FOR CARRYING OUT THE INVENTION

Based on best modes for carrying out the invention, the presentinvention will next be described in further detail. The followings arethe course of technical endeavors and the findings, which have led tothe present invention.

With a view to developing a super carburizing process for causingultrafine carbide to precipitate in a surface layer portion of a memberby using low-pressure carburizing facilities, the present inventorscarried out a thorough investigation on possible relations between theconcentration of carbon in the surface layer portion and various heatingand cooling conditions and the precipitation form of the ultrafinecarbide in the surface layer portion and the microstructure of thematrix. Concerning improvements or the like in strain by heat treatmentwhile assuming members such as gears and axles, research and developmentwas also conducted from many directions. An aim was then set at theestablishment of a novel process for super carburizing and low-strainquenching, which can achieve both of mutually-conflicting properties ofproviding a member with higher performance by super carburizing andminimizing a deformation, distortion or the like of the member whilebalancing them at high levels.

Upon applying super carburizing to a surface layer portion of steel(member) , the most important point is to have ultrafine carbideprecipitated as much as possible in a surface layer portion of themember through the optimal combination of the primary treatment and thesecondary treatment. In the control of the formation of the ultrafinecarbide, carburizing and quenching facilities also play an importantrole. In the present invention, a variety of developments were conductedwhile using low-pressure carburizing facilities that compared withconventional carburizing facilities, permit a variety of controlpromptly with higher accuracy as to the concentration of carbon in themember, the repetition of carburizing treatment/diffusion treatment, anddiverse temperature conditions, heating conditions and cooling rate(quenching) conditions for heating, soaking, carburizing, quenching andthe like of the member.

Described specifically, a variety of investigations were conducted onthe heating, soaking, super carburizing, diffusion and cooling(quenching) conditions of a member during the primary treatment tofirstly reduce the deformation or strain of the member at the stage ofthe primary treatment. In the secondary treatment as the next step,carburizing and quenching (cooling) conditions are important to permitadjustments or the like in the precipitation of ultrafine carbide andthe grain size of austenite in the carburized layer. Specifically, ithas been found that in the secondary treatment, the deformation orstrain of a member by the heat treatment can be minimized by controllinga range, in which the ultrafine carbide precipitate in a surface layerportion of the member, to 10 to 30% in terms of effective case depthpercentage and further by converting an outermost surface layer portioninto an ultrafine crystalline structure.

The term “effective case depth percentage” as used herein means a ratio(t/T) of a precipitated depth (t) of ultrafine carbide existing in anoutermost surface layer portion of a member to an effective case depth(T) of the member after completion of the secondary treatment (includingthe tempering treatment at 180° C.). It is to be noted that the term“effective case depth” means a distance from a surface of a hardenedlayer, which is still in a quenched state or has been tempered at atemperature not exceeding 200° C., to the position of a critical depthof a Vickers hardness (HV) of 550 as measured by the Method of MeasuringCase Depth Hardened by Carburizing Treatment for Steel (JIS G0557).

Next, the term “precipitated depth of ultrafine carbide” means themaximum depth, where the ultrafine carbide exists, from the outermostsurface layer portion of the member as determined by an analysis underan optical microscope or an electron microscope. To facilitate thediscrimination of the ultrafine carbide, the member is analyzed in astate of being etched with an etching solution such as 5% nital etchingreagent.

The vacuum carburizing (low-pressure carburizing) facilities for use inthe present invention are equipped with a carburizing and heatingchamber including a treatment furnace which is sectionally controllableat different pressures of from 200 to 2,000 Pa, and are available on themarket. Conventionally-available vacuum carburizing facilities are allusable in the present invention. As the primary treatment in the presentinvention, the member is heated and soaked to a predeterminedtemperature in the furnace of the facilities, and to raise theconcentration of carbon in the surface layer portion of the member to orhigher than the eutectoid carbon concentration, the member is thenquenched at an appropriate cooling rate. In the subsequent secondarytreatment, the carbide is caused to precipitate in an ultrafine form inthe surface layer portion of the member, optionally followed byadditional carburizing treatment as needed.

According to the primary treatment in the process of the presentinvention, steel to be treated (member) is heated and soaked to anaustenite region of from 900 to 1,100° C., carburizing is conducted suchthat the carbon concentration of a surface layer portion becomespreferably 0.8 wt. % or higher, and from the thus-carburized state,quenching is then conducted at an optimal cooling rate. Optimal coolingconditions are to evenly cool the member at a cooling rate of from 3 to15° C./sec over a temperature range of from the carburizing temperature(the temperature in the austenite region) to the A₁ transformationtemperature or lower, preferably to 400° C. or lower. By this cooling,ultrafine carbide is caused to precipitate in the surface layer portionof the member so that a structure composed primarily of martensite isformed in the surface layer portion. The term “ultrafine carbide” meansan M₂₃C₆ type carbide formed as a result of bonding of carbide-formingelements such as Cr and Mo in Fe₃C (cementite) or steel with carbondissolved in supersaturation.

In the secondary treatment, the non-carburized portion (interior) of themember is heated and soaked to a range of from an austenizingtemperature to the austenizing temperature+80° C., preferably to a rangeof from 10 to 70° C. above the austenizing temperature, and is thenrapidly quenched to effect precipitation of ultrafine carbide such thatthe carbon concentration of the surface layer portion becomes preferably0.8 wt. % or higher, more preferably 1.0 to 2.0 wt. %. It is preferredto apply, in parallel with the precipitation of the ultrafine carbide inthe surface layer portion, additional carburizing treatment to thesurface layer portion to promote the precipitation of the ultrafinecarbide in the surface layer portion, and from the state that the carbonconcentration of the matrix has been adequately adjusted, to furtherconduct rapid quenching.

The temperature of the final quenching after the secondary treatmentvaries depending on the pretreatment conditions, that is, whether thefinal quenching is after the heating and soaking or after the heating,soaking and additional carburizing. The rapid quenching can be conductedat the temperature after the pretreatment or at a temperature raised orlowered relative to the temperature of the pretreatment. In other words,the temperature of the final quenching after the secondary treatment canbe set at a level commensurate with the quality of heat treatment suchas the hardness and microstructure required for the member.

With a view to establishing optimal conditions for super carburizing,the present inventors conducted a detailed investigation on the carbonconcentrations upon heating, soaking and carburizing and diffusion andvarious cooling (quenching) conditions with respect to the primarytreatment in which super carburizing is applied to a surface layerportion of a member in low-pressure carburizing facilities and thesecondary treatment in which ultrafine grains of carbide are caused toprecipitated in the surface layer portion. As a result, it was succeededin obtaining a super carburized, quenched member having a carbonconcentration of preferably 0.8 wt. % or higher, more preferably from1.0 to 2.0 wt. % in a range of from 10 to 30% in terms of the percentageof an effective case depth (t/T) in an outermost surface layer portionand having a three-layer structure consisting of a superultrafine grainlayer of No. 10 or greater austenite grain size, a fine grain layer andan ultrafine grain layer in this order from the outermost surface layer.It has been found that the super carburized, quenched member isminimized in deformation or distortion after the treatment and that thecorrection of a strain, which has been unavoidable in the conventionalsuper carburizing, can be obviated or can be readily conducted comparedwith the conventional process.

EXAMPLES

Based on certain Examples, the present invention will next be describedin further detail.

Machine structural steels (materials) shown in Table 1 were provided.Those materials were subjected beforehand to normalizing treatment at900° C. and were then machined to prepare stepped round-bar test piecesof φ30/φ25/φ20×L 300 mm, respectively. As carburizing and quenching ofeach test piece, the primary treatment of the super carburizing step inthe present invention was conducted using facilities which permittedheating and carburizing at a low pressure and also permitted oilhardening and high pressure gas cooling.

It is to be noted that steel grades 1 and 2 are carburizing, quenchingsteels as specified under the JIS, steel grade 1 is SCM420,chromium-molybdenum steel, and steel grade 2 is SCr415, chromium steel.MAC14 as steel grade 3 is a grade for a commercial product developed bya steel maker, and is steel developed by increasing the Cr content incomparison with the above-described two steel grades and further addingMo element with a view to causing M₂₃C₆ type ultrafine carbide toprecipitate upon super carburizing (the primary and secondarytreatments).

TABLE 1 Used Steels and Their Chemical Components (wt. %) Steel grade CSi Mn P S Cr Mo 1 SCM420 0.20 0.30 0.75 0.019 0.025 1.10 0.20 2 SCr4150.16 0.35 0.78 0.021 0.019 1.05 0.02 3 MAC14 0.15 0.27 0.53 0.020 0.0222.50 0.38

Table 2 summarizes the results obtained by experimenting in various wayseffects of the cooling rate on the states of carbide to be precipitatedin surface layer portions of test pieces and the deformations of thetest pieces by heat treatment through the primary treatment in thepresent invention. As conditions for the primary treatment, supercarburizing of each test piece was conducted by the heat cycle shown inFIG. 1 such that subsequent to heating and soaking, an effective casedepth of 0.5 mm would be achieved. Described specifically, supercarburizing and diffusion treatment of each test piece were alternatelyconducted at 950° C. for about 70 minutes, respectively, such that thecarbon concentration of the surface layer portion of the test piece inits final state would be controlled at about 1.5 wt. %. From a statethat the carbon concentration of the surface layer portion of each testpiece was in supersaturation, quenching of the test piece was conductedunder the corresponding cooling rate condition shown in Table 2 toinvestigate the shape and size of the carbide in the surface layerportion of the test piece and the microstructure of the surface layerportion of the test piece.

To determine the deformations and strains of the above-described steelgrades by the primary treatment, stepped round-bar test pieces(φ30/®25/φ20×L 300 mm) of the respective steel grades were provided astest pieces. In a state of being supported at opposite ends, each testpiece was analyzed for a runout at its axial central part to investigatea relationship between the cooling rate and the axial of the test piece.

TABLE 2 Relationships between Cooling Conditions for Primary Treatmentand Precipitation Form of Carbide and Runout Cooling MicrostructureSteel rate Shape and size of of surface Runout · TIR Ex./Comp. Ex. No.grade (° C./sec) carbide layer portion (mm) Comp. Ex. 1 SCM420 1 Flaky,3-10 μm F + P + B 0.45 Ex. 2 Same as 12 Granular, 0.5-5 μm M + T 0.17above Comp. Ex. 3 Same as 20 Granular, ≦2 μm M + γ 0.38 above Comp. Ex.4 SCr415 1 Flaky, 3-10 μm F + P 0.40 Ex. 5 Same as 4 Granular, 0.5-5 μmM + T 0.15 above Comp. Ex. 6 MAC14 1 Granular + flaky, 5 μm F + P + B0.38 Ex. 7 Same as 7 Flaky, 2-7 μm M + T 0.20 above TIR: TotalIndicating Reading

The signs shown in the table and analysis methods of the propertiesshown there will now be described below.

-   1) The cooling rate indicates an average cooling rate at the axial    central part of each test piece from the quenching temperature of    950° C. after the completion of the carburizing and diffusion for    the test piece to 400° C.-   2) The shape and size of carbide was observed under a scanning    electron microscope.-   3) Abbreviations for microstructures    -   F: ferrite, P: pearlite, B: bainite, T: troostite, M:        martensite, y: retained austenite.-   4) The radial runout indicates a runout of a test piece, which was    mounted on a both-end supporting, runout measuring instrument, as    measured at a central part of the test piece by a dial gauge.

In each of the comparative examples shown as Test Piece Nos. 1, 4 and 6in Table 2, the cooling rate during the cooling was as low as 1° C./secso that the carbide precipitated in the surface layer portion consistedprimarily of a network of carbide formed of carbide flakes bondedtogether and the matrix was in the form of an slack quenching structureof ferrite, pearlite and bainite. As a consequence, those comparativeexamples were all large in radial runout and deformation. Thecomparative example shown as Test Piece No. 3, on the other hand, wassubjected to rapid cooling equivalent to conventional oil quenching (20°C./sec) . Its surface layer portion contained a very small amount ofprecipitated carbide, and had a structure quenched from a high carbonstate that carbon was in supersaturation. That comparative example waslarge in radial runout and deformation.

When the cooling rate was 4 to 12° C./sec as in each of the examples asTest Piece Nos. 2, 5 and 7 (the present invention), ultrafine carbideprecipitated in a large amount, and moreover, microstructures appearedas nuclei for the ultrafine carbide, leading to improvements in thedeformation and distortion (runout) of the test piece as the outstandingserious problems of super carburizing. Described specifically, comparedwith slow cooling that cooling is slow or rapid quenching that coolingis fast in contrast, the radial runout of each of the test piecesaccording to the present invention was of approximately a half level ofthe radial runouts in the rest of the examples, thereby realizing asubstantial reduction in radial runout. From these results, the coolingrate during the quenching in the primary treatment is optimally 3 to 15°C./sec.

Table 3 shows the results obtained by using representative ones of thetest pieces subjected to the primary treatment shown in Table 2,applying the secondary treatment in various ways to the representativetest pieces to cause ultrafine carbide to finally precipitate in theirsurface layer portions, and investigating the carbon concentrations,states of precipitated carbide, microstructures, crystal grain sizes,etc. in their surface layer portions and the radial runouts of the testpieces. As conditions for the secondary treatment, the heat cycle shownin FIG. 2 was followed, the soaking temperature was selectively set atthree levels of 800° C., 850° C. and 900° C., all above the A₁transformation temperature, and subsequent to the heating and soaking,additional carburizing was also conducted at the same time to achieve acarbon concentration higher than the eutectoid carbon concentration as atechnique for further raising the carbon concentrations in the surfacelayer portions and also increasing the amounts of precipitated ultrafinecarbide through the secondary treatment.

The subscript “n” in (carburizing/diffusion)n or (additionalcarburizing/diffusion)n in FIGS. 1 through 3 means the number ofrepetitions of carburizing or diffusion in the corresponding step, andis set in commensurate with the quality required for each member. In thecase of Test Piece No. 2 shown as an example in Table 2, for example, nwas set at 8 (n=8), and in the case of Test Piece No. 2-2 shown as anexample in Table 3, on the other hand, n was set at 5 (n=5).

TABLE 3 Relationships between Treatment Conditions for SecondaryTreatment and Precipitation Form of Carbide and Runout Austenite grainsize and three-layer Temperature of structure secondary Precipitation ofOutermost Ex./ Steel treatment Additional carbide surface Three Comp.Ex. No. grade (° C.) carburizing Shape Amount layer layers Comp. Ex. 2-1SCM420 900 Applied Ultrafine A little ≧10 Included Ex. 2-2 Same as 850Applied Ultrafine Adequate ≧10 Included above Comp. Ex. 2-3 Same as 800Applied Flaky Excessive ≧10 Included above Ex. 5-1 SCr415 850 AppliedUltrafine Adequate ≧10 Included Ex. 5-2 Same as 850 Not appliedUltrafine A little 6-8 Not above included Ex. 7-1 MAC14 850 AppliedUltrafine Adequate ≧10 Included Ex. 7-2 Same as 850 Not appliedUltrafine A little 6-8 Not above included Carbon Effective Percentage ofEx./ Steel concentration case depth effective case Runout · TIR Comp.Ex. No. grade Microstructure (%) (mm) depth, t/T (%) (mm) Comp. Ex. 2-1SCM420 M + γ 1.6 0.52  5 0.35 Ex. 2-2 Same as M 1.5 0.48 25 0.14 aboveComp. Ex. 2-3 Same as M + F 1.5 0.46 20 0.20 above Ex. 5-1 SCr415 M 1.60.50 18 0.16 Ex. 5-2 Same as M 1.2 0.46 10 0.23 above Ex. 7-1 MAC14 M1.4 0.53 25 0.21 Ex. 7-2 Same as M + γ 1.1 0.48 15 0.25 above TIR: TotalIndicating Reading

[Analysis Method of Carbon Concentration Surface Layer Portion]

Using each of the test pieces (φ30/φ25/φ20×L 300 mm), chips werecollected by lathe turning from the surface layer portion to the 0.05 mmdepth of its φ25 mm portion, and the carbon concentration of the surfacelayer portion was determined by a chemical analysis.

From Table 3, the Test Piece No. 2 series indicate effects on theprecipitation form of carbide and others when the secondary treatmenttemperature was varied, and the Test Pieces No. 5 and No. 7 seriesindicate effects on the precipitation of ultrafine carbide and the finalcarbon concentrations in the surface layer portions depending on whetheror not the additional carburizing was applied in the secondarytreatment.

Concerning the secondary treatment temperature (which may herein afterbe called “the additional carburizing temperature”), the temperature of900° C. employed for Test Piece No. 2-1 involves a problem in that thecarbide in a surface layer portion dissolves to lead to a reduction inthe overall precipitation of carbide grains and also to an increase inthe radial runout of the test piece. With the secondary treatmenttemperature of 800° C. employed for Test Piece No. 2-3, carbide flakesprecipitate at grain boundaries in the surface layer portion, and thecore portion of the member is quenched incomplete. Test pieces,therefore, develop variations in radial runout. From these results, theoptimal temperature for the treatment that causes ultrafine carbide toprecipitate in a surface layer portion by the secondary treatment canpreferably be a temperature equivalent to the A₃ transformationtemperature+10-70° C., which is determined by the composition of themember (before the carburizing treatment).

As to whether or not the additional carburizing treatment is applied inthe secondary treatment, the application of the additional carburizingtreatment has been recognized, as evident from the results of Test PieceNos. 5-1 and 7-1, to bring about the advantageous effect that carbideprecipitates in an ultrafine form, to say nothing of an improvement inthe concentration of carbon in the surface layer portion. As a reasonfor the advantageous effect, it may be contemplated that, as the carbonin the surface layer portion precipitate as carbide and theconcentration of carbon in the matrix becomes lean, the replenishment ofcarbon to the surface layer portion by the additional carburizing couldpromote the new formation of ultrafine carbide, such as Fe₃C and M₂₃C₆,and nuclei thereof.

As shown in FIG. 4, it has also been found that in the member subjectedto the additional carburizing treatment, the austenite grain size of theoutermost surface layer portion is reduced to an ultrafine grain size.The term “ultrafine grain size” corresponds to an austenite grain sizeof No. 10 or greater as measured by the carburized grain-size testingmethod in JIS-G0551, “Method of Testing Austenite Grain Size for Steel”.A significant characteristic feature has also been discovered in that athree-layer structure formed of fine grains and ultrafine grains isformed extending toward the inside. Paying attention to a relationshipbetween the austenite grain size and the carburized layer, the grainsizes of the outermost surface layer portion greatest in the amount ofprecipitated ultrafine carbide, the carburized layer portion (fine grainportion) located inside the outermost surface layer portion and theultrafine grain portion located still inside the fine grain portion arein a relationship of A≧C≧B, in which “A”, “C” and “B” stand for theoutermost surface layer portion, the ultrafine grain portion and thefine grain portion, respectively. Incidentally, the austenite grain sizeof a surface layer portion in conventional carburizing is generallyequivalent to No. 7 or 8. In the present invention, the surface layerportion has a grain structure of the characteristic three-layerstructure which does not appear in the conventional carburizingtreatment.

As an advantageous effect of such an ultrafine grain layer, it has asignificant characteristic feature in that the toughness of a hardenedsurface layer, said toughness having been a concern about conventionalcarburized members, can be improved and high toughness can also beimparted to the carburized layer itself in addition to the feasibilityof higher contact pressure as a characteristic feature of the presentinvention, and therefore, is extremely effective for providingcarburized members with still higher strength from now on.

Table 4 shows effects of the percentage of an effective case depth of acarbide layer precipitated in super carburizing according to the presentinvention on various properties. Various test pieces were prepared byproviding SCM420, JIS steel for machine structure, as a material,subjecting the material to normalizing treatment at 900° C. beforehand,and then machining the resultant material. The super carburizing of eachtest piece was conducted by the heat cycle of primary treatment andsecondary treatment shown in FIG. 3. Each treated test piece wasanalyzed and investigated for pitting life, impact strength, distortionby heat treatment, etc. Concerning effects of the carbon concentrationof the outermost surface layer portion of each test piece shown in Table5, the test piece was treated by the heat cycle shown in FIG. 3 in asimilar manner as the various test pieces in Table 4, and the carbonconcentration and the like of the treated test piece were investigated.

The adjustment of the precipitation depth of carbide in Table 4 waseffected primarily by the control or the like of the carburizing timeand carbon concentration, and the adjustment of the carbon concentrationof the outermost surface layer portion in Table 5 was effected bycontrolling the process gas flow, treatment time and the like uponrepeating carburizing and diffusion in the primary treatment andsecondary treatment in accordance with a program calculated beforehand.Process gases for low-pressure carburizing include propane, acetylene,ethylene and the like. Among these, most popular and economical propanewas used. As an inert gas upon diffusion, on the other hand, nitrogengas was used. Further, the rapid quenching in the secondary treatmentwas conducted by oil. As an alternative, the rapid quenching can also beconducted by high pressure gas which makes sole or mixed use of gasessuch as N₂, He and H₂.

TABLE 4 Effects of the Percentage of Effective Case Depth on Strength,Durability and Distortion by Heat Treatment. Carbon Rolling Percentageconcentration fatigue of effective of the outermost life Impact casedepth, Carburizing surface layer (number of strength Roundness Ex./Comp.Ex. Sign t/T (%) time (min) portion (%) rotations) (J) (μ) Comp. Ex. A 580 1.0 6.5 × 10⁶ 118 29 Ex. B 10 104 1.5 1.1 × 10⁷ 110 31 Ex. C 20 1191.7 2.1 × 10⁷ 105 39 Ex. D 30 134 1.9 2.3 × 10⁷ 98 50 Comp. Ex. E 40 1492.0 2.2 × 10⁷ 67 65

TABLE 5 Effects of the Carbon Concentration of Outermost Surface LayerPortion on Strength, Durability and Distortion by Heat Treatment CarbonRolling concentration of fatigue life Impact Ex./ outermost surfaceCarburizing (number of strength Roundness Additional Ref. Ex. Sign layerportion (%) time (min) rotations) (J) (μ) carburizing Ref. Ex F <0.8 805.3 × 10⁶ 56 30 Not applied Ex. G 1.0 80 1.5 × 10⁷ 87 32 Same as aboveEx. H 1.5 80 2.0 × 10⁷ 69 37 Same as above Ex. I 1.0 96 1.9 × 10⁷ 116 35Applied Ex. J 1.5 133 2.4 × 10⁷ 111 39 Same as above Ex. K 2.0 130 2.6 ×10⁷ 98 53 Same as above

-   1) The percentage of effective case depth indicates the ratio (t/T)    of the depth (t) of an ultrafine carbide layer to a case depth (T)    of 550 HMV or greater in terms of micro-Vickers hardness.-   2) The rolling fatigue life indicates the number of repetitions of    rotation until occurrence of pitting under the below-described    conditions.    -   Contact pressure: 3 GPa, rotation speed: 1,500 rpm, slipping        ratio: −40%, oil temperature: 80° C.-   3) The impact strength indicates destructive energy as measured    using a Charpy test piece.-   4) The roundness indicates the amount of a deformation of the inner    diameter of a ring in the X-Y direction as measured by a profile    measuring instrument while using as the ring a test piece in a ring    form of φ100(φ80)×15 t.

A description will now be made about effects of the percentage ofeffective case depth on the rolling fatigue life. When an ultrafinecarbide layer was as shallow as 5% in terms of the percentage ofeffective case depth as in the comparative example represented by thesign A, it is considered that the amount of precipitated ultrafinecarbide itself was small and therefore, that the treated test piece didnot have temper softening resistance, which is characteristic to supercarburizing, and was low in pitting toughness. In the case of thecomparative example represented by the sign E in which the percentage ofeffective case depth was 40%, the high hardness range was broadened,resulting in a problem that the impact strength was reduced, and withrespect to a deformation by heat treatment as determined in terms ofroundness, there was also a tendency toward increased distortion. Fromthese results, the percentage of effective hardened depth in aprecipitated carbide layer is optimally in a range of from 10 to 30%.

A description will next be made about effects of the carbonconcentration of the outermost surface layer portion shown in Table 5 onthe pitting life. It is considered that the signs H, J and K, in each ofwhich the carbon concentration of the outermost surface layer portionwas high, were superior in pitting life and that in the cases of thesigns G and I in each of which the carbon concentration was 1%, that is,lower compared with the former signs, they were somewhat inferior inpitting life. When the carbon concentration of the outermost surfacelayer portion is lower than 0.8 wt. % as in the sign F shown as areferential example, the test piece was significantly inferior inpitting toughness. Namely, the greater the amount of ultrafine carbideprecipitated in the outermost surface layer portion and the higher thecarbon concentration of the outermost surface layer portion, the betterthe pitting life. Accordingly, the carbon concentration of supercarburizing can be set preferably at 0.8 wt. % or higher in the presentinvention.

Regarding the upper limit to the carbon concentration throughcarburizing, no particular problem arose up to 2.0 wt. %. An increase incarbon concentration to a still higher level in excess of 2.0 wt. %involves a potential concern that precipitation of carbide flakes may befacilitated and the impact strength and deformation by heat treatment ofthe test piece may tend to become disadvantageous. It is, therefore,necessary to set the carbon concentration of the outermost surface layerportion at a level commensurate with properties required for the member(test piece).

A description will next be made about effects of the additionalcarburizing treatment in the secondary treatment in the signs I, J and Kon the pitching life, impact strength and deformation (strain) by heattreatment. Compared with the signs G and H in each of which the carbonconcentrations was similar but the additional carburizing was notapplied, the signs I, J and K varied less in all the properties and werebetter. As a reason for this advantage, it can be contemplated that theadditional carburizing treatment may stabilize the carbon concentrationof the matrix and may also promote the formation of ultrafine carbide inthe outermost surface layer portion, the carburized layer itself may beconverted into a dense and well-balanced structure, and the qualityavailable through the heat treatment may be thoroughly stabilized.

From the above-described various analysis results, it is desired, asoptimal treatment conditions in the process of the present invention, toemploy machine structural steel as a member, to conduct supercarburizing as a combination of the primary treatment and the secondarytreatment in low-pressure carburizing facilities to treat the memberunder optimal heating and cooling conditions, and then to control thefinal step such that the depth of precipitated carbide falls within therange of from 10 to 30% in terms of the percentage of effective casedepth and the carbon concentration of the surface layer becomes 0.8 wt.% or higher.

INDUSTRIAL APPLICABILITY

As appreciated from the above-described series of results, the presentinvention can provide an absolutely novel, super carburized,low-distortion quenched member and its production process. According tothe present invention, machine structural members such as gears and axlemembers can be provided with higher strength and can be used underhigher contact pressure, thereby making it possible to materialize withlow distortion the needs for various members of higher strength, higherperformance, lighter weight and smaller size, such as members requiredto have low distortion, rotary sliding or reciprocal sliding membersequipped with bearing structures, and members required to have highcontact fatigue resistance and high abrasion resistance under highcontact pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] Heat cycle of the primary treatment.

[FIG. 2] Heat cycle of the secondary treatment.

[FIG. 3] Heat cycle of the examples.

[FIG. 4] Optical micrograph (magnification: ×100) of Test Piece No. 2-2in Table 3.

1. A process for producing a super carburized, low-distortion quenchedmember, which comprises a primary treatment of heating a steel memberfor a machine structure to a temperature within an austenite region byvacuum carburizing (low-pressure carburizing) to have carbon dissolvedat least at a eutectoid carbon concentration of a surface layer portionof said member and then quenching said member at a cooling rate of from3 to 15° C./sec from said temperature within said austenite region to atemperature not higher than an A₁ transformation point to have at leastone of ultrafine carbide and nuclei of said carbide formed in saidsurface layer portion of the said member; and a secondary treatment ofsubsequently heating and soaking said member to a temperature withinsaid austenite region and then conducting rapid quenching to haveultrafine carbide precipitated in a range of from 10 to 30% in terms ofeffective case depth percentage in an outermost surface layer portion.2. A process according to claim 1, wherein in said secondary treatment,additional carburizing treatment is applied to said surface layerportion of said member.
 3. A process according to claim 2, wherein insaid secondary treatment, said ultrafine carbide is caused toprecipitate in said surface layer portion of said member to form astructure composed primarily of martensite and containing a mixedstructure of troostite, retained austenite and the like in parts thereofsuch that said outermost layer portion (a portion A) of said layer, alayer portion (a portion B) inner than said portion A and a layerportion (a portion C) inner than said portion B are in an order of A≧C≧Bin terms of the fineness of austenite grain size.
 4. A super carburized,low-distortion quenched member comprising a surface layer portion of astructure composed primarily of martensite and containing a mixedstructure of troostite and retained austenite or the like in partsthereof, wherein in said surface layer, an outermost surface layer (aportion A), a layer (a portion B) inner than said portion A and a layer(a portion C) inner than said portion B are in an order of A≧C≧B interms of the fineness of austenite grain size.